Misfires are problematic in two aspects. On the one hand, they increase the toxic gas content in the exhaust gas already with a misfire frequency of several percent of all ignition strokes and, on the other hand, they endanger the catalyzer with unburnt fuel which is then combusted in the catalyzer. This danger begins when the misfire strokes amount to approximately 10% of the total number of ignition strokes assuming nothing is undertaken to counteract the effect of the misfires. One conventional measure for avoiding damaging effects is to interrupt the metering of fuel to a cylinder having misfires.
Various methods for determining misfires are known such as measuring the ignition signals, the measurement of the combustion pressure, the investigation of noise signals, the optical check for the presence of ignition sparks, the measurement of exhaust gas temperature in the outlet elbow or the investigation of crankshaft speeds within pregiven crankshaft angle sectors.
Most of these methods are quite complicated and require considerable computations. Especially costly microcomputers must be used to obtain acceptable computation times notwithstanding the high complexity of computation.